For me at the moment, the one I fly most is the Stratocruiser with COTS. Just a sublime sim experience
I bought COTS when it came out but didn't fly with it for ages, a bit intimidated I guess, but took the plunge finally, and its fantastic. With COTS I fly the Strat a lot more often now.
Next is the first Accusim a/c I bought, which was the P47. Big, loud, tough and has long range. Love the inertia engine start. After I bought this one, I knew FSX for me wouldn't be the same again. So it holds a special place for me
Least flown is the Cub (great plane, has Heidi, but just a bit too simple and sooo slow for me).
I do like flying around OrbX Pacific North West in this one.
The Spitfire is awesome, but a pain to live with
(worth the effort though). And its a Spitfire, what more can I say.
The B-17 is superb. Great having the crew there, and its a B-17!!! Big grin factor on this one.
Don't own the P-40, not interested I'm afraid (but thats just me).
I already have a couple of fighters, and I'm waiting for the Mustang (NOT the H version).
So to wrap up, no doubt once you've bought one, you'll buy more, its guaranteed my friend.
So don't be overly worried about which one you'll buy first, I'm sure you won't be disappointed.
Had to edit this 'cos I forgot Accu-Feel, I know its not an Accusim plane but it does make a big difference to non Accusimmed a/c.